This invention relates to regulation of the output voltage of electrical power sources and, more particularly to the regulation of DC electrical power sources which include a generator having an exciter field coil with the output of the generator being proportional to current in the exciter field coil.
DC electrical power systems offer several advantages over AC systems in aerospace applications. For example, DC systems eliminate reactive voltage drops in feeder lines at high power levels and are compatible with electromechanical actuators. Also, DC power may be developed by a wide speed range generator and simple rectifier thereby eliminating the constant speed drive or converter required for constant frequency AC systems. Furthermore, off line power conversion components and circuits can operate directly from relatively high voltage DC with very high frequency converters replacing 400 hertz transformers and rectifiers. A typical 270 volts DC aircraft generating system includes a wide speed range generator having a multiple phase AC output which is rectified by a full wave bridge rectifier and filtered by a capacitor to produce the DC output. A voltage regulator senses the DC output voltage and controls the excitation of the generator to maintain the output voltage.
In such systems, the time constant of the output filter capacitor and load resistance can vary over several orders of magnitude as the load is changed. At the extreme case of no load, the output capacitor has no discharge path at all. A load removal transient on the system will naturally cause an overshoot in the output voltage. The overshoot voltage becomes trapped on the filter capacitor which holds the output high for a time dependent only on leakage currents. If the regulator senses DC output voltage and the bridge rectifier is not conducting, the regulation control loop is broken. Under those conditions, the voltage regulator senses the high output voltage and reduces the generator excitation to zero, attempting to reduce the overshoot. In effect, the generator goes to sleep. The generator AC output will not start to increase until the DC output falls below the regulation set point. At this time, the regulator begins to increase excitation to the generator. Excitation current can only increase at a rate limited by the forcing voltage and inductance of the exciter field winding. As generator output increases, it will overshoot the regulation point again and a limit cycle may result.
If the load is suddenly reapplied while the generator is asleep, the only source of power will be the output filter capacitor. The output voltage will be severely depressed until the generator output increases.
Compromises may be made in the system performance to improve voltage stability. Time constants in the regulator may be chosen to eliminate the limit cycle but only at the expense of good load transient response. Even a very slow responding system will still have the sleeping generator problem at no load. A bleeder resister may be added across the DC output to bring the output voltage down more quickly, but at the expense of efficiency. The required bleeder current may be several percent of the rated output current. A bleeder current of 2% will cause a 2% loss in efficiency, more than doubling the losses in the rectifier.
It is therefore desirable to regulate DC voltage systems in a manner which limits output voltage transients resulting from the operating scenario described above.